Magnetic storage devices are used to store data on a magnetic storage medium through the use of a transducer that writes and reads magnetic data on the medium. For example, a disc storage device is generally adapted to work with one or more magnetic recording discs that are coaxially mounted on a spindle motor of the device for high-speed rotation. As the discs rotate, corresponding transducers, i.e., read/write heads, are moved across the surfaces of the discs by an actuator assembly to read and write digital information on the discs.
Given the general desire to store ever-increasing amounts of digital information, designers and manufacturers of magnetic storage devices are continually attempting to increase the bit density of magnetic storage media. In a magnetic recording disc, this means increasing the areal density, i.e., the number of tracks on a disc and/or the linear density of bits along a given track. One manner in which areal densities have been increased has involved the use of bit-patterned media (BPM).
In BPM, the magnetic recording surface is patterned to provide a number of discrete, single-domain magnetic islands (usually one island per bit) separated from each other. During recording and readback of data to and from the bits, the separation of the bits greatly limits exchange coupling there between. As a result, bits can be positioned close together when using BPM while preventing interference between the bits. Servo information is often included on the BPM in order to provide positioning information for a servo control system. To this end, during a writing operation on a BPM, a write or recording head can be precisely positioned over a given data array, e.g., data track, to magnetize the bits thereon, where such bits are often referred to as dots. Thus, for example, during the rotation of a magnetic recording disc, the head can be carefully aligned over the dots it passes in order to facilitate data recording and data readback processes.
However, even with such general alignment between the head and disc dots, BPM has presented further challenges to disc-drive operation. One of these challenges has involved synchronizing the write timing of a disc storage device with the discs and their existing dots. Based on preliminary modeling results, the window for successful writes to a disc can generally be found to be within 30% of the length of the bits. Given a general density of 1 terabit per square inch, such 30% length translates to about 4 nm. Depending on the RPM and radii of the drive operations, this could further translate to writing periods on the order of 10 ps. Consequently, the general writing operation permits limited opportunity for feedback to the disc storage device. Further, it is commonly agreed that the write head (or writer) is not able to provide position feedback by itself. In other words, the writer does not function as a sensor so as to register its position relative to the ‘writable’ dots on the disc. Thus, there are challenges in locating a sensing apparatus to facilitate correction of the write timing synchronization and in providing timely feedback of position information to the disc storage system. Embodiments of the present invention are focused on addressing these challenges.